New driving control system with haptic feedback: Design and preliminary validation tests

Transportation Research Part C 33 (2013) 22–36

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Transportation Research Part C journal homepage: www.elsevier.com/locate/trc

New driving control system with haptic feedback: Design and preliminary validation tests Jorge Juan Gil a,b,⇑, Iñaki Díaz a, Pablo Ciáurriz a, Mikel Echeverría a a b

CEIT, Paseo Manuel Lardizábal, 15, E-20018 San Sebastián, Spain TECNUN, University of Navarra, Paseo Manuel Lardizábal, 13, E-20018 San Sebastián, Spain

a r t i c l e

i n f o

Article history: Received 26 January 2012 Received in revised form 5 March 2013 Accepted 3 April 2013

Keywords: Haptic feedback Drive-by-wire Automobile technology

a b s t r a c t This paper presents a new mechatronic system that combines the capabilities of the steering wheel, the throttle and brake pedals in a single all-encompassing device. A two degreeof-freedom mechanism allows controlling all driving functionalities together in a very ergonomic and original way. The system uses drive-by-wire technology with haptic feedback for an outstanding driving experience. The device has been tested in a simulation platform, showing similar performance to conventional set of steering wheel and pedals, and very good acceptance among users. This work also surveys current drive-by-wire systems in the automotive industry and the use of haptic technology to assist drivers. Ó 2013 Published by Elsevier Ltd.

1. Introduction Drive-by-wire technology in the automotive industry replaces the traditional mechanical and hydraulic control systems (e.g., steering column) by specific electronics to control a wide range of vehicle operations such as accelerating, braking or steering. There are several different types of drive-by-wire systems depending on the mechanical component it is replacing: throttle-by-wire, brake-by-wire and steer-by-wire. In any type of these by-wire systems, typical hydraulic and mechanical components are replaced by sensors that record information and pass data to a computer, which converts the electrical energy into mechanical motion (Chiappero and Back, 2002). This technology has multiple advantages: safety, for example, can be improved by providing computer controlled intervention of vehicle controls (automatic brake in dangerous situations, electronic stability control, etc.). The elimination of the steering column contributes to safety since, in case of frontal collision, this mechanism may force the hand wheel into the driver, causing injuries and fatalities. Ergonomics can also be improved by controlling the amount of force and range of movement necessary to control the systems. In fact, the driver can simply use a game-like joystick or controller to drive the vehicle. Furthermore, the number of moving parts in the vehicle are also significantly reduced, thus simplifying vehicle design and reducing weight and mechanical maintenance costs. Regarding the disadvantages of drive-by-wire systems, the development costs of these components are higher than conventional ones due to their greater complexity. Nevertheless, the greatest disadvantage is the fear traditional drivers have of system failures that may cause a runaway vehicle (similar to computer crashes at home). Because of the complexity of these systems, people worry about potential electronic malfunctions in sensors and computers, leading to unknown vehicle behavior. To that respect, manufacturers believe that new generations of drivers will be more accustomed to the use of gaming and robotic technology, thereby solving this drawback. This paper surveys different drive-by-wire solutions in literature, and proposes a new vehicle-control paradigm using a mechatronic system that is designed to control steering, throttle and braking functions with a unique hand-manipulated ⇑ Corresponding author at: CEIT, Paseo Manuel Lardizábal, 15, E-20018 San Sebastián, Spain. Tel.: +34 943 212800; fax: +34 943 213076. E-mail addresses: [email protected] (J.J. Gil), [email protected] (I. Díaz), [email protected] (P. Ciáurriz), [email protected] (M. Echeverría). 0968-090X/$ - see front matter Ó 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.trc.2013.04.004

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device. The proposed mechanism additionally incorporates haptic technology that allows taking advantage of the human sense of touch (Srinivasan, 1995). Haptic systems can apply forces, vibrations, and/or motions upon the user with a high degree of control over parameters such as the frequency, duration and amplitude of the signals. These interfaces have proven to be very efficient for high performance human–machine interactions, exploring the capabilities of the human sense of touch as an advanced communication channel. Haptic feedback can be used both to improve the driving experience and to warn the driver about dangerous situations. Drive-by-wire systems remove tactile feedback felt by drivers in steering wheels and pedals due to the direct mechanical transmission between the wheels and the system. Although this results in a reduction of undesired noise and vibrations, many users claim that with these systems they can not ‘‘feel’’ the road anymore. To overcome this effect, haptic feedback can be incorporated into drive-by-wire systems. Regarding safety issues, in certain cases the visual and auditory senses of the driver may be overburdened due to the high amount of information displayed (radio, navigation-system, etc.), and therefore these channels might be inadequate to process warning signals. As an alternative, haptic feedback can be used to alert the user about warning situations. This feedback usually has a small number of competing demands and can be perceived simultaneously with visual and auditory signals (Sklar and Sarter, 1999).

2. Drive-by-wire technology applied to the automotive industry In this section, a chronological overview of several concept cars that feature drive-by-wire technology is given. Focus is on innovative vehicles, but alternative drive-by-wire technologies that are considered relevant are also described.

2.1. Joystick-driven concept cars The replacement of the steering wheel with a joystick is not new. In 1959 General Motors presented a Chevrolet Impala equipped with a 2-DoF (degrees of freedom) joystick that was located on the side of the driver’s door. More than 30 years later, in 1991, Saab developed a joystick steered concept car, based on a Saab 9000. In this case, the joystick was located at the site of the shifter. Turning the joystick yields a change in wheel angles and this change is also fed back to the user by adjusting the response of the joystick. The steering ratio is speed dependent. The car is equipped with a conventional accelerator and an automatic gearbox. The first fully drive-by-wire concept car using side stick controllers was developed by Daimler-Chrysler. Not only steering, but also accelerating and braking were commanded by the joystick. A Mercedes–Benz SL-based demonstrator (sometimes called Daimler-Chrysler R129 in the literature) was built to test the technology and the feasibility of abandoning conventional steering wheels and pedals (Huang, 2004). Moving the joystick forward makes the car accelerate, moving the joystick backward activates the brakes or—after stopping—activates the reverse gear. This concept is the forerunner of several vehicles with side sticks: the Mercedes–Benz F200 Imagination (1996) and the Mercedes–Benz SL 500 (1998). The SL 500 is equipped with two side sticks (from Fokker Control Systems) for accelerating, braking and steering. The existence of two joysticks allows the driver to choose which hand to use, or to use both hands. The joysticks in early versions had two axes. In a second version, acceleration and braking were commanded by applying pressure to the joystick—thus not moving it forward or backward—in order to have a clearer distinction between steering and speed control, thereby increasing steering accuracy. If no pressure is applied, the car maintains the current speed. The sideward deflection of the joystick is related to the actual wheel angle. Within the ‘‘Project Z’’ framework, the Bolduc Technology Group and related companies Electronic Mobility Controls (EMC) and AEVIT Services Company (ASC) equipped, in 2005, a Nissan 350Z with a centrally placed drive-by-wire joystick for steering and with another one for acceleration and braking (placed left). The steering joystick also enables switching gears and accessing secondary control functions (lights, etc.). ItalDesign Vadho Concept (2007). This hydrogen powered concept car has joysticks for steering, an electrical throttle and an electrical brake pedal. The position of the joysticks and pedals is adjustable, whereas the driver’s seat is fixed. The Honda Puyo fuel cell concept car (2007) is equipped with joystick steering and pedals for acceleration and braking. The car can turn 360° in place. Honda earlier performed tests with an Accord with full drive-by-wire joystick control. Revolution Motors Dagne EV (2008). This is an electric vehicle with three wheels in which all of them are driven by electromotors. The vehicle leans into the corner like a motorbike. Steering, acceleration and braking take place via a single joystick. The DLR FASCar was developed by the German Aerospace Center (DLR), based on a Volkswagen Passat. The development of FASCar is part of a project called HAVEit (2008–2011), in which automated driving technologies and new HMI’s have been researched. Both a passenger car (FASCar) and a truck (EWBtruck) are presently being developed. In the current version of FASCar, the driver can always override the assistance systems. The driver can also use a side stick instead of the steering wheel and pedals. Control of the Mercedes–Benz F-Cell Roadster concept car (2009) is not through a conventional steering wheel and pedals, instead a joystick mounted in between the seats provides all the vital control functions like steering, acceleration and braking.

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In the Toyota FT-EV II ultra-compact electric vehicle (2009), steering wheel and brake and gas pedals have been replaced by a pair of retro-styled drive-by-wire joystick controls. These joysticks duplicate one another’s movements so it does not matter which one is pushed, pulled or turned to control the car. The Rinspeed UC? [sic] is an electric vehicle developed in 2010 by the Swiss entrepreneur Frank Rinderknecht. The UC? uses SpaceDrive technology from the German company Paravan. SpaceDrive is a vehicle control system which uses electronic and digital input devices, one example of which is a joystick. The UC? is driven by means of its four-way joystick control system which works in a similar way to that operated by a jet pilot. In order to perfect the driver’s handling of the vehicle and to provide feedback on the driving characteristics, the joystick gives the operator haptic information about the road and driving performance. 2.2. Control yokes for drive-by-wire cars Although steering-by-wire technology can be implemented on conventional hand wheels, this section presents only those innovative cars that use alternative control wheels, similar to flight yokes, that include the possibility of accelerating and braking. The Bertone/SKF Filo car (2001) features drive-by-wire steering, accelerating, braking and gear shifts using a steering pod that is placed on a bar coming from the center console. The steering pod contains motorcycle-type twist-grip throttles and bars for braking. The feedback to the driver is provided as a function of the loads acting on the steering rack, by an appropriate high-torque motor. The same SKF drive-by-wire device is used in the Bertone/SKF Novanta concept car (2002). The GM Hy-wire hybrid car (2002)—combustion engine and fuel cell—with drive-by-wire technology features a steering wheel similar to the ones found in aircraft. In order to accelerate, the driver should twist (either of) the handgrips on the steering wheel. The brakes are activated by squeezing the handgrips. Changing direction is done by pulling the handgrips up or down. Citroën has also developed a special steering wheel that incorporates control of direction, acceleration and braking using by-wire technology. It was first presented on the Citroën C-Crosser (2001) and Citroën C-Airdream (2002) concept cars, while an enhanced device is included on the Citroën C5 by Wire (2005). Acceleration is realized by pressing either of the pads using the thumbs. Braking is achieved by applying pressure to the braking pads on the steering wheel. Fig. 1 shows the main drive-by-wire features of the cars described in the last two sections.

Fig. 1. Overview of drive-by-wire concept cars.

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2.3. Applications for disabled people Joystick-driven cars have already been available for disabled people for several years. In general, they are customized vehicles, adapted to fit the specific needs of the driver. Various joystick systems can be connected to the existing steering system in the car. Usually, the joysticks do not provide the user with force feedback. Several companies offer the possibility to equip a car with joysticks for steering, or even for both steering, acceleration and braking. Examples of companies that install and develop such steering systems are Paravan GmbH, Electronic Mobility Controls LLC (also used in the Nissan 350Z described in Section 2.1) and Ahnafield Corp. The joysticks send signals to electrical motors that turn the steering wheel. Depending on personal preference either a single two-axis joystick is used or two separate one-axis joysticks. Some of the joysticks provide custom control of the response of the joystick, i.e. its rotational stiffness in all four directions can be adjusted. However, none of them provides controllable force feedback. A newly developed system, Joysteer, contains two joysticks—both of them for steering, accelerating and braking—and does provide force feedback with respect to the steering. It has been developed at Bern University of Applied Sciences, Switzerland, but commercialized by Bozzio AG. The Joysteer contains two control systems to provide redundancy in case of technical failure. Another joystick-driven car for handicapped people is presented in Wada and Kameda (2009). The joystick operation in back and forward direction controls acceleration and deceleration of a car, while left and right direction controls the steering of the vehicle. The prototype includes three different driving modes. The first one uses the steering wheel and the foot pedals in the same way as a normal car. The second method uses the steering wheel and a mechanical knob linked to the pedals for accelerating and braking. In the latter, all movements are controlled by a joystick. The lateral movement of the joystick is coupled to the steering wheel, so that the steering commands given to the joystick are reproduced in the steering wheel. Regarding research about joystick driving by disabled people, the Swedish National Road and Transport Research Institute (VTI) has performed a number of research projects on the use of drive-by-wire joysticks for steering, acceleration and braking in automobiles (Peters and Östlund, 2005). The focus of this research is on the use of joysticks by disabled people, but tests have also been performed on people with normal hand/arm functionality. Joysticks with two rotational degrees-of-freedom have been researched as well as joysticks which allow for one rotation and one translation. Such research has found that in driving experiments time delay occurs in the steering system, since the joystick can be moved faster than the steering system can react. Moreover, it is difficult to learn how to brake using angle-controlled braking without feedback. Also—without information feedback to the joystick—interference between the primary driving tasks easily occurs and lack of tactile feedback limits the speed at which lateral movements can be performed. Lastly, when driving using a joystick, external disturbances (e.g., wind or road surface) can easily influence movements of the joystick, especially in the case of disabled people sitting in a wheel chair that is placed in the car. In the experiments carried out in a driving simulator it was found that decoupling longitudinal and lateral control (i.e. steering and accelerating/braking) has a positive influence on driving behavior and performance. Adding active feedback improves performing lane change maneuvers, but when driving on rural roads it does not improve control. Moreover, disabled people with limited arm/hand strength found active feedback less pleasant to use.

2.4. Similar technologies in other applications Besides passenger cars, other types of vehicles are equipped with joystick control. Moreover, joystick motion control is also used in applications that have no relation with the automotive sector at all. In this section, some examples of these categories will be given. Aircrafts. From the 1980s on, modern aircrafts use digital fly-by-wire systems (starting with the Airbus 320 series and followed by Boeing with the 777 series). Analog fly-by-wire systems were already introduced thirty years before. Advantages of fly-by-wire systems are that pilots can get used to flying new aircrafts relatively easily and the systems only allow control within the safety limits of the aircraft. Aircrafts usually have threefold or fourfold redundancy regarding the flyby-wire system. Military vehicles. The U.S. Army is investigating the influence of non-standard control devices (e.g., joysticks) on human driving behavior. Tests have been performed using a simulator in order to get more insight into nonlinear steering ratios. According to the website of the Bolduc Technology Group, EMC drive-by-wire technology has been used on several military vehicles (as well as on boats). Forklifts. Current developments have led to research on drive-by-wire force feedback to the steering wheel, plus a socalled ‘‘observe-by-wire’’ feedback. The feedback to the steering wheel is thus not only based on the side forces on the tire, aligning torques, friction and inertia, but also incorporate a force based on the distance between the fork and the object to be lifted. Initial simulations indicate an increase in productivity, since it becomes easier to position the forklift correctly with respect to an object. Wheel loaders. The Komatsu WA800-3 has a so-called ‘‘Advanced Joystick Steering System’’. This fully hydraulic system enables the driver to perform precise steering movements with the over 100.000 kg vehicle. The single joystick also enables changing gears and switching some secondary functions.

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3. Haptic feedback in drive-by-wire devices Beyond the state of the art in drive-by-wire technology many studies have been conducted to determine the optimal haptic interaction methods for active driving joysticks. This section overviews the most relevant conclusions found, which, together with related work presented in the previous section, have served as a basis to the development of the new drive-bywire system. 3.1. General remarks According to Kember and Staddon (1989), systems with high-order dynamics, that is, with the capacity of rapid change, can be controlled with considerable advantage with a rigid, force-controlled joystick. A more extensive investigation of suitable interface properties dependent on the control objective can be found in Rühmann (1978), where it is remarked that if a process is being controlled by prescribing its position, a viscously damped joystick is desired; however, if a process is controlled by prescribing its velocity, a spring-centered or viscously damped joystick is desired, and if the acceleration is prescribed, an isometric (rigid) joystick is desired. These conclusions are valid for so-called ‘‘compensation control’’ and passive joysticks. Driving a car is usually considered as compensation control (as opposed to so-called ‘‘following control’’) (Bubb, 1993). In compensation control, people appear to be better in prescribing the velocity. In following control people appear to be better in prescribing the position (Bubb, 1978). The translation of these conclusions to active joysticks in cars (i.e. with haptic feedback) is made in Huang (2004). For several kinds of feedback (force/torque/angle/position and their derivatives) the stability, apparent stiffness and suitability for application in cars are researched. The choice for a specific combination has a very extensive impact on the interaction between driver and vehicle and on vehicle handling in general, so enough attention must be paid to this election. The coherence between optic and haptic feedback is also an issue that has to be taken into account. Very related to the choice of the controlled quantities, is the choice for either measuring the joystick deflection and feeding back a force to the joystick, or measuring the force/torque exerted by the driver and feeding back a joystick deflection. Each strategy has its benefits and drawbacks (Huang, 2004). Other general remarks in Huang (2004) are: (i) the inertia of the joystick should be as low as possible, and (ii) when controlling a two-axis joystick, errors in longitudinal direction are usually larger than errors in lateral direction due to the anatomy of the hand. 3.2. Steering The maximum angle of a joystick is generally in the order of 20° to both sides, compared to about 720° to both sides in the case of a steering wheel. This large difference suggests the need for a speed dependent steering ratio (Östlund, 1999). If the front wheel deflection is made dependent on the ratio of the joystick deflection to the square of the speed, then a constant ratio between joystick deflection and the lateral acceleration felt by the driver is obtained. This change in steering ratio can be applied, for example, for speeds higher than 30 km/h. The use of speed-dependent low-pass filtering plus a speed-dependent steering ratio is called ‘‘progressive control’’ (Östlund, 1999). However, low-pass filtering might be dangerous in a case of rapid (or emergency) maneuvers at higher speed, since the desired movement could be canceled by the filter (Östlund, 1999). The steering ratio of the Daimler-Chrysler side sticks depends on the force applied in the sideward direction of the joystick and on the driving speed (Huang, 2004). The applied force is filtered using a low-pass filter with a 0.16–0.64 Hz cut-off frequency, depending on the current amount of feedback. The curvature of the direction in which the car drives is fed back to the joystick (i.e. converted into a joystick deflection). As a result, the stick becomes stiffer at higher driving speeds. In Huang (2004), it is argued that at low speeds the curvature of the direction in which the car drives should be fed back and at higher speeds the yaw velocity of the car. Systems like ESP and DSC do not only control the yaw velocity of the car, but also take the slip angle into account, since only controlling the yaw velocity may lead to large slip angles. This is also important in the case of joystick steering. Moreover, it is said that the steering ratio can be dependent on, e.g. driving-speed, joystick deflection, joystick deflection speed, yaw velocity of the car and vehicle load. By controlling the yaw velocity (instead of the wheel angle) it is possible to automatically cancel side wind and road disturbances. It is proposed, however, to cancel only high-frequency disturbances but not low-frequency disturbances, so that the driver still receives information about the current driving circumstances. 3.3. Accelerating and braking In general, feedback on driving speed has been shown to be stable whereas feedback on acceleration can yield instability in the driver-vehicle-control loop (Huang, 2004). In the Daimler-Chrysler side sticks, a linear relationship is established between the force applied by the driver and the acceleration of the car. This linear profile is speed-dependent. A low-pass filter is added (cut-off frequency at 2.4 Hz) and a threshold of 2 N or 0.3 N m has to be overcome. A control system cancels the influence of the slope of the road and of air resistance, i.e. it makes sure that the acceleration remains zero, that is, constant

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velocity, as long as the driver does not touch the joysticks. Isometric (rigid) sticks are used, since feedback of the driving speed would not be noticeable at low acceleration. For angle-controlled braking without haptic feedback, the aim is to have a linear transmission between joystick deflection and ‘‘braking effect’’. Since braking is a power-controlled process, the dynamics of the braking system have to be taken into account (Östlund, 1999,). When the driver brakes very hard, his/her body tends to move forward due to inertia. In case of joystick systems for disabled people, usually the braking function of a joystick is directed forward, so that inertia does not yield reduced braking (Östlund, 1999). 3.4. Reverse driving In principle there are two possibilities for reverse driving: a direction compatible system or a functional assignment. In the direction compatible case, the joystick is moved forward to drive forward and moved backward to brake. After stopping, moving the joystick backward will make the car drive backward and moving the joystick forward will make the car brake. In the functional case, moving the joystick forward makes the car accelerate (forward or backward dependent on the gear selection). Moving the joystick backward always activates the brakes. Note that the other way around can also be programed, as is the case for most systems for disabled people (Östlund, 1999): moving the joystick backward makes the car accelerate (forward or backward) and moving the joystick forward always activates the brakes. Tests in static simulators showed that the direction compatible assignment is preferred by most people. However, real driving tests with the Daimler-Chrysler side sticks showed that the functional assignment is better. Moreover, it is presumed that the direction compatible assignment has a higher rate of accident risk (Huang, 2004). 3.5. Assisting the driver The improvement of safety and pleasure on the road are driving forces of technological advances in the automotive industry. In particular, new Advanced Driver Assistance Systems (ADAS) have considerable potential for making the experience of driving more relaxed and safer, by means of mitigating human errors (Amditis et al., 2007). One of the critical elements of ADAS is communication with the user, which has to be clear and efficient, but at the same time it must not overload the driver’s attention and perception resources. In fact, the second and third design goals of the European Statement of Principles on the Design of HMI (ESoP) indicate that the allocation of driver attention while interacting with displays and controls must be compatible with the attentional demand of the driving situation, and specifically that the system must not distract or visually entertain the driver (2007/78/EC,). In order to lessen the visual load, many concepts of ADAS use haptic displays. One feature of haptic systems is that the part of the body that receives the information is usually the same one that manipulates the interface, and thus action and feedback can be coupled. This advantage has been applied to manual interaction in the steering wheel, through active steering systems that automatically modify the wheel angle or torque resistance, in order to attenuate yaw disturbances, or for ‘‘shared control’’ between vehicle and driver in path following tasks (Ackermann et al., 1999; Steele and Gillespie, 2001). A similar approach for the lower limb has led to the development of active haptic pedals, which exert a variable counterforce depending on vehicle dynamics or surrounding traffic, in order to manage congestions (van Driel et al., 2007; Brookhuis et al., 2009) or control speed (Adell and Várhelyi, 2008,). However, while continuous haptic gas-pedal feedback is effective for car-following, it can be insufficient in more dangerous situations, when the distance to the leading vehicle is small, and quick corrective control actions must be taken to prevent collision (Mulder et al., 2008). Warning signals in the form of tactile vibrations or pulses have been largely tested in pedals (Lloyd et al., 1999; Tijerina et al., 2000; Martens and van Winsum, 2001; de Rosario et al., 2010), steering wheel (Tijerina et al., 2000; Suzuki and Jansson, 2003; Jordan et al., 2007), and driver seat (Lee et al., 2004; Stanley, 2006; Jordan et al., 2007). What part the vibration is associated with normally depends on the type of warning and the expected action. It is exerted on pedals to compel braking events, in speed or collision warnings, and on the steering wheel for warnings related to lateral events that need steering actions, such as a risk of lane departure. Seat vibration is used for both types of warnings, and also for other purposes, like providing orientational cues in a navigation system (van Erp and van Veen, 2004). This type of directional haptic feedback may also be applied to other functions, such as calling attention to the windscreen or the rear-view mirror, by a vibrating belt with tactors on the driver’s abdomen and back (Ho et al., 2005). 4. New haptic drive-by-wire device Based on previous literature, this section presents a new haptic drive-by-wire interface that has been designed, built, programed and preliminarily tested. Its main purpose is allowing the driver to give steering, accelerating and braking commands. The system does not include buttons for gear changing (it is assumed that the vehicle includes an automatic gear selection system). Although the technical concept was chiefly designed for people with lower-extremity impairment (Dominguis et al., 2011), the device is intended for providing any user with an enhanced driving experience. The designed interface couples the steering, accelerating and braking functions in a very different style to that found in the related work, so it could neither be properly classified as a joystick nor as a control yoke. The design paradigm overcomes many limitations of common joysticks for a proper driving experience. Additionally, the new device is perfectly suited for haptic feedback capabilities.

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Following subsections describe the former proof-of-concept prototype designed for lab testing. First, the original design solution to couple all driving functionalities into a unique hand-held device is presented. Afterwards, the control algorithms for its maneuverability and the haptic laws are described. The validation tests into a driving simulator are reported in Section 5. 4.1. Hardware description The design of the mechanical interface meets several functional and ergonomic requirements: all-in-one solution (steering, accelerating and braking commands) using a single hand (right hand by default), reversible design (adaptable to lefthanded people), attractive appearance and user-friendly solution. The sketch of the final design is shown in Fig. 2. An arrow points to the forward direction of vehicle motion. The selected material (aluminum) and the use of a nerved structure meet one of the mechanical aspects mentioned in Section 3.1, that is, low inertia (approximately 6.94 g m2 in the steering movement). The haptic drive-by-wire device has two active degrees of freedom. The steering command is correlated with the angle of the lever hs (see Fig. 2). The acceleration command is introduced by rotating the handle towards a positive angle ha. This solution decouples better the lateral and longitudinal control (i.e. steering and accelerating) than conventional joysticks. Additionally, user’s inertia during accelerating or braking does not interfere a priori with the driving command. The main characteristics of the two active joints are summarized in Table 1. The lateral deflection of the lever doubles conventional joysticks range in order to provide more versatility to the prototype. However, the final workspace is reduced by the software (this is a general parameter that can be tuned). The workspace of the handle is also restricted based on maximum comfortable wrist rotation (Diffrient et al., 1981). Each joint is commanded by an electric actuator. The first one is a 150 W DC motor (Maxon RE40-148877) connected to the lever by means of a cable transmission with a reduction stage of 10. This type of mechanical transmission presents high reversibility, high stiffness, no backslash and near zero friction, allowing the system to reproduce realistic haptic sensations. A high resolution incremental encoder (Quantum devices QD145-05/05-5000) is attached to the motor for measuring joint rotation. The second actuator is enclosed within the handle itself. It is an 11 W DC motor (Maxon RE-max 24-222053) attached to the handle through a planetary gearhead with two stages and a reduction of approximately 29. The resulting apparent inertia of the rotor attached to the gearhead, 0.39 g m2, is still moderate and does not degrade the haptic feedback.

Accelerating

Steering

Forward direction of vehicle motion

Fig. 2. Haptic drive-by-wire device (left) and sketch with active joints definition (right).

Table 1 Technical specifications of the active joints. Parameter

Lever

Handle

Max. rotation angle (hardware) Max. rotation angle (software) Angular resolution Max. continuous torque Peak torque Reduction ratio

±40° ±30° 0°000 2600 2.008 N m 4.016 N m 10:1

Continuous From 0° to 45° 0°010 2700 0.373 N m 0.467 N m 729:25

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Notice that the selected motors and sensor have been chosen for the testing prototype and not for its direct use in a real car; that is, the device is dimensioned to reach a very wide range of torques, enough to test different control possibilities and haptic feedbacks. Regarding the braking command, two different strategies have been analyzed with this concept prototype. The first one uses the opposite direction of the accelerating axis (Fig. 3 left), while the second one uses a passive brake lever (Fig. 3 right). Since it is quite difficult to determine a priori which is the best braking configuration, the validation tests have also been used for discussing this issue (Section 5.2). 4.2. Control algorithms and haptic feedback For each active joint two main variables are controlled: (i) the command signal to the vehicle and (ii) the haptic torque fed back to the user. The first one is a normalized value that ranges from 1 to 1 (emulating a conventional joystick signal) and the second one is a torque definition. This last choice (measuring deflection and feeding back torque) is the common architecture in impedance-type haptic systems. However, the torque definition does not only depend on the position of the device. Some other parameters of the vehicle are also considered. 4.2.1. Steering The angle of the steering lever hs and the angle of the wheels hw are not linearly mapped. The desired value for the wheels hdw is weighted by two sensitivity ratios (r1 and r2):

hdw ¼ r1 r 2

hs hs max

ð1Þ

The first ratio r1 introduces a dependency of the sensitivity on the angle of the steering lever (less sensitive around the center position). The second ratio r2 introduces a dependency of the sensitivity on the vehicle speed v (less sensitive as the vehicle speed increases) based on Östlund (1999), but tuned differently. Fig. 4 shows these two sensitivity ratios. Although r1 is a linear function of the normalized steering angle, for a given vehicle speed the commanded value hdw , Eq. (1), is a quadratic function. This kind of nonlinear behavior is highly recommended for levers with a reduced angular range compared to conventional steering wheels (Andonian et al., 2003). The torque ss restored to the user by the steering lever has three components: one proportional to the angle of the lever (felt as if the lever is attached to the center position by means of a spring with stiffness coefficient Ks), a viscous component with damping coefficient Bs, and a third term based on the difference between the desired wheel angle and the true wheel angle.

ss ¼ K s hs  Bs

  dhs þ jw hdw  hw dt

ð2Þ

These two definitions, Eqs. (1) and (2), imply a bilateral communication between the haptic device and the vehicle: the haptic device sends the desired angle for the wheels hdw to the vehicle, and the vehicle reports the actual orientation hw.

Fig. 3. Braking strategies: using the handle (left) and using a passive lever (right).

Fig. 4. Sensitive ratios depending on the angle of the lever hs and vehicle speed v.

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4.2.2. Accelerating Regarding the accelerating command, the opening of the throttle valve hdv is a normalized value from 0 to 1 directly controlled by the rotation of the handle ha. This is comparable to using a conventional gas pedal. The opening of the throttle valve is linearly increased up to a certain maximum angle hamax. This limit is set to 45° based on the maximum comfortable wrist rotation (Diffrient et al., 1981).

hdv ¼

ha ha max

ð3Þ

The torque exerted to the driver consists of two resistive terms. The first one is proportional to vehicle speed v and the second one is a viscous force:

sa ¼ jv v  Ba

dha dt

ð4Þ

Again, Eqs. (3) and (4) imply bilateral communication: hdv is sent to the vehicle and v is received from it. Notice that some constants (Ks, Bs and Ba) have physical meaning—they are stiffness and viscous coefficients—and can be easily tuned. Other constants (jw and jv) have a less intuitive meaning and should be experimentally tuned. 4.2.3. Braking The passive brake lever (Fig. 3 right) does not require any haptic feedback (it has a real spring) and the normalized braking command (from 0 to 1) is proportionally mapped from the rest position to the maximum deflection of the lever. If the handle is used to brake (Fig. 3 left) the braking command is proportionally mapped to the handle’s turn in the braking direction from 0° up to a maximum angle hb max which is 18°. Within this range, a virtual spring with stiffness Kb is simulated:

sb ¼ K b hb

ð5Þ

5. Driving simulation tests The functionalities of the haptic device have been tested by multiple users in a static part-task driving simulation platform. The system is validated by comparing its performance against a conventional set of steering wheel and pedals. Future work will validate its functionalities in a real vehicle. 5.1. Simulation platform The driving simulator consists of several programs that are running simultaneously. Firstly, the rFactor,1 a computer racing simulator developed by Image Space Incorporated. rFactor has the ability to run any type of four-wheeled vehicle through multiple scenarios, but more importantly, it allows a high degree of control and monitoring over many car parameters, and a realistic dynamic response to such variables. Secondly, the haptic control loop that governs the new driving device runs at 1 kHz in a dSPACE DS1104 board. This data acquisition board records the encoders information from the device, and applies a proper actuation to its motors. Data communication between both applications is performed by means of an rFactor plug-in, which allows reading the telemetry data from rFactor and using it in other applications. An example of the plug-in is available from the developers of rFactor and it has been extended and compiled with dSPACE commands in order to enable communication from rFactor to the dSPACE board. As explained in Section 4.2, to provide real haptic feedback, and not to display simple passive springs and dampers, the haptic loop requires information from the vehicle (in this case from the rFactor program), and so does the rFactor to display the user’s interaction with the new driving system. Table 2 reports the values of all the parameters defined in Section 4.2 that have been used in the control and haptic algorithms. These parameters have been selected to obtain comfortable driving. Although one of the advantages of the proposed system is that the driver can easily modify all the parameters depending on his/her preferences (for example ‘‘softer’’ or ‘‘harder’’ for a more sportive driving), they remain constant during these preliminary tests. Later, in Section 5.5, an additional experiment is carried out to analyze the influence of these parameters on the controllability of the new driving command. 5.2. Tests design and hypothesis A set of driving tests have been performed using the simulation platform (Fig. 5) to analyze the usability of the device. Three different driving configurations have been explored: the first one uses a gaming steering wheel with pedals from LogitechÒ (G25 Racing Wheel), and the last two employ the drive-by-wire system, but with a different braking configuration (Fig. 3). The following abbreviations are used to identify the three different driving configurations: 1

http://www.rfactorcentral.com/.

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J.J. Gil et al. / Transportation Research Part C 33 (2013) 22–36 Table 2 Parameter values for the haptic algorithms. Parameter

Variable

Value

Steering stiffness Steering damping Wheel feedback Speed feedback Accelerating damping Braking stiffness

Ks Bs

0.55 N m/rad 0.1 N m s/rad 4.5  104 N m 0.005 N m s/m 0.015 N m s/rad 0.7 N m/rad

jw jv Ba Kb

Fig. 5. Simulation platform using the haptic device (left) and using a set of steering wheel and pedals (right).

– W: Conventional steering wheel with pedals. – B1: Drive-by-wire system, braking with the accelerating handle. – B2: Drive-by-wire system, braking with the passive brake lever. The objective variable to measure the performance of the driving commands was the time to complete a specific circuit of rFactor. The selected track was the ‘‘Essington Park GP Circuit’’ (4.012 km and 12 turns), which was considered long enough to detect significant discrepancies in terms of completion times when using efficient vs. inefficient driving configurations. Thus, the hypothesis is that the conventional steering wheel with pedals should achieve the best experimental results (faster driving and therefore shorter completion times), while the drive-by-wire system may differ significantly (or not) with respect to this conventional driving (longer completion times are expected). A group of 16 participants took part in these experiments, 5 women and 11 men, with ages varying from 23 to 50 years old (average of 31). All of them were right handed and reported normal tactile and visual functions. All subjects had a driver’s license and driving experience. Most of them also had prior experience with haptic applications, but none of them with the new drive-by-wire device. All the participants drove a specific rally car (Renault Clio Sport) in automatic shift mode, and with the stability, braking, and traction controls of rFactor activated. In order to avoid the influence of non-desirable effects such as the learning effect and the ‘‘well traveled road effect’’ on the results, the tests with each driving command were conducted in different days and also in different order for each user. Before each experiment, the users had two warm-up laps to get used to the device and the circuit. Afterwards, they were asked to complete three laps to the circuit as fast as they were confident. During these three laps, several data were recorded, including velocity, jerk, lap times, partial times in each sector, average and top speeds, average use of the throttle (in percentage to total time) and any remarkable incident or event, such as collisions, off-track excursions, etc. In case of collision or off-track excursion, those laps were not taken into account for the results. The key variables for comparing the driving commands were the best lap time (no matter if it was the first, the second or the third one) and the jerk measures.

5.3. Measured results Fig. 6 shows the box plot of the best lap times (min:sec) of all the users for each configuration (W, B1 and B2). The boxes contain the middle half of the data points. The lines inside the boxes are the median values. The vertical lines cover the range of all values, except outliers (asterisks). A solid line connects the mean values (crossed circles). It can be seen that, as expected, W obtained the best mean lap time (02:01.32), but only 2.17 s faster than B1 and 2.75 s faster than B2. In general, all results among different configurations were very similar.

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Fig. 6. Best lap times for the three different configurations.

The statistical significance of the two drive-by-wire configurations (B1 and B2) compared to the conventional set of steering wheel with pedals (W) for the measure under analysis—the best lap time—can be investigated by using a linear model that includes as factors the users and the drive-by-wire configurations. This way, the effect of the user—which can perform the test with very different performance—is removed and it is possible to state whether the experiments reflect the same pattern (in this case that the best lap time is always faster using W despite of the driver from the statistical point of view). The results of this study show that the effects of B1 and B2 on the best lap time are significant, t(30) = 2.293, p = 0.029 and t(30) = 2.901, p = 0.007 respectively, but their overall effect is quite small (an increase of 1.55 ± 0.68% and 1.97 ± 0.68% respectively). A similar performance of the different driving commands can also be seen in Fig. 7, which shows a comparison of the top and average speeds among the three configurations. Using hand wheel and pedals (W) the speeds were slightly higher, and their corresponding standard deviation smaller, but again very similar to those obtained with the drive-by-wire system. As expected, the average use of the throttle (triangles connected by a solid line in Fig. 7) was higher for B2. This is due to the fact that both acceleration and braking are uncoupled for B2, and therefore it was possible to accelerate and brake at the same time in this configuration. Smoothness is another interesting driving parameter that can be analyzed. A quantitative measure of smoothness can be obtained by computing the rate of change of the vehicle’s acceleration, which is the third derivative of position and also known as ‘‘jerk’’ (Schot, 1978). This parameter can be examined over time by calculating the root of the mean of the squares (RMSs) of instantaneous jerk measurements. A small RMS jerk means a smooth maneuver, and therefore, it can be used as a measure of the ride comfort (Post, 2011). Two different RMS jerk values were obtained, one for the longitudinal direction of the vehicle (Fig. 8 left) and another for the lateral one (Fig. 8 right) recorded during the best lap. The box plots in Fig. 8 show quite similar performance in the jerk measures for the three driving configurations. The statistical significance of the two new driving configurations (B1 and B2) with the conventional set of steering wheel with pedals (W) is also investigated for the jerk values by using a linear model that includes as factors the users and the cited new driving configurations. In the longitudinal jerk, no significant differences were found: t(30) = 1.693, p = 0.101 for B1 and t(30) = 0.233, p = 0.818 for B2. However, in the lateral jerk, both B1 and B2 do exhibit a significant influence: t(30) = 2.298, p = 0.029 and t(30) = 3.265, p = 0.003 respectively. Therefore, from these preliminary experiments, it can be stated that the new driving configurations achieve smaller values in the lateral jerk. However, it is important to note, that the resulting

Fig. 7. Top speed, average speed and % throttle.

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Fig. 8. Values for the longitudinal and lateral jerk.

smoothness is not perceived by the participants in this driving simulation platform. Thus, these results in ride comfort should be confirmed with in-vehicle tests. 5.4. Subjective evaluation of the new mechatronic device After the last driving test, the subjects were asked to assess the drive-by-wire prototype regarding five different aspects, including overall rate, satisfaction, ease of use, comfortableness and security. The subjective rating covers a 7-point Likert scale, from 1 (worst) to 7 (best) for each of the adjectives proposed. The results can be observed in Fig. 9. In general, the new device received good scores in all aspects. Participants were very enthusiastic with the new approach, and enjoyed the new driving paradigm. Only the score obtained for comfortableness was slightly lower than the average. Questioned about this fact, participants stated that the only drawback they found was that by the end of the race they felt the wrist a little bit tired since they had been continuously accelerating with their hand. Except for motorbike riders, users are more relaxed accelerating with their feet rather than with their hand. Notice also that the driving task proposed for the experiments required users to complete the race as fast as possible, thus demanding high accelerations, which would not occur so significantly in real routes. Regarding the two design solutions adopted for the braking command, while results presented in Fig. 6 showed no significant difference in performance, the majority of the users (62.5%) preferred braking configuration B1 (using the accelerating handle in the opposite direction). 5.5. Influence of haptic parameters on the controllability of the vehicle The previous experiments were carried out with certain predefined haptic parameters (Table 2) specified by the authors for the new driving command. These parameters were subjectively selected to create a comfortable driving sensation. However, their contribution to vehicle controllability has not yet been discussed.

Fig. 9. Subjective evaluation (mean and standard deviation).

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International standard ISO 7401:1988 provides some test methods for measuring the lateral transient response of road vehicles. Since the overall handling behavior is difficult to assess, the lateral transient response can be used to analyze the controllability of the vehicle relative to the steering command. One of the methods proposed by ISO 7401:1988 consists of introducing a random steering input to obtain the frequency response of the vehicle (lateral acceleration or yaw velocity vs. steering command angle). Among other requirements, the input signal is generated by a test driver for 900 s, during which time the vehicle response is measured. Lateral acceleration during the test should remain below 4 m/s2 and vehicle speed should be constant at 80 km/h.

Fig. 10. Lateral frequency response of the vehicle (yaw velocity vs. steering command angle) for haptic parameters Ks (N m/rad), Bs (N m s/rad) and jw (N m).

6 4

Gain (dB)

2 0 −2 −4 −6 −8 −10 −12 0.2

Drive−by−wire device Steering wheel

1

4

1

4

Phase (deg)

0 −20 −40 −60 −80 0.2

Frequency (Hz) Fig. 11. Lateral frequency response of the vehicle (yaw velocity vs. steering command angle) using the new drive-by-wire device and the steering wheel.

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Following this procedure, several experiments were carried out in the driving simulator to analyze the influence of the haptic parameters on vehicle controllability. A different driving scenario was selected in order to allow driving in a straight lane. The same driver performed all the tests. In each test, the parameters for the haptic feedback were modified by taking the following values: Ks = 0.2, 0.55, 1.1 N m/rad, Bs = 0.05, 0.1, 0.25 N m s/rad and jw = 2.4  104, 4.5  104, 9  104 N m. Middle values were equal to those in Table 2 and they were considered default values for the experiments. A total number of 9 tests were performed, 3 per parameter (keeping the other parameters at their default values). In addition, another test was performed with the steering wheel. Fig. 10 shows the frequency response of the vehicle’s yaw rate to the steering input. It can be observed that haptic parameters can be changed within a wide range of values to meet drivers’ preferences without compromising vehicle controllability. It is interesting to note that the dynamic response of the vehicle using the steering wheel from LogitechÒ is much more different than any recorded response using the drive-by-wire device. For sake of simplicity, Fig. 11 only presents the lateral responses using the steering wheel (dashed line) and the new device using the default haptic parameters (solid line). In both cases the input of the experiment was the steering command angle in radians. The reduced range of movement of the driveby-wire device, ±30°, compared with the steering wheel, ±90°, is the main cause for the different starting gain levels. Moreover, the gain of the dynamic response using the new device is higher for all frequencies. As a consequence, with the new command, a smaller change of steering angle hs will produce a larger displacement of the wheels, no matter how fast the movement is. 6. Conclusion Manufacturers are constantly striving to improve existing technology to develop cutting-edge transmission systems that can offer drivers an enjoyable driving experience. Drive-by-wire systems are a very promising technology in the automotive field. We must bear in mind that the absence of a mechanical connection between the command devices and the driving elements should not be taken as a risk. Moreover, this leaves room for further vehicle improvement in terms of weight, controllability, safety, etc. The use of drive-by-wire controls with haptic feedback also allows the system to restore the user tactile information as if she/he were manipulating an interface mechanically connected to other parts of the vehicle. Thus, the driving experience remains without confusion, but new assistance features could be implemented by using this new haptic channel (e.g., warning signals). This paper firstly presents a review of existing drive-by-wire systems, as well as the design guidelines proposed in the literature. Afterwards, a new drive-by-wire command with haptic feedback and its control algorithms are described. The concept design takes advantage of previous research and proposes an innovative configuration to decouple the driver’s lateral movement for steering with wrist turn for accelerating/braking. The mechatronic solution was tested on a driving simulation platform. The resulting performance was good and similar to what was obtained with a conventional hand wheel with pedals. Objective and subjective evaluations were carried out as well as a vehicle controllability analysis. In the near future, this command will be installed in an electric vehicle for its final validation in real driving tests. Acknowledgements This work has been supported by the project MARTA, led by Ficosa International S.A. and funded by the Centro para el Desarrollo Tecnológico Industrial (CDTI) for the 3rd CENIT Programme, as a part of the INGENIO 2010 Programme of the Spanish Government. The authors would like to acknowledge the fruitful discussions with Dr. Ángel Rubio. References 2007/78/EC, 2007. Commission Recommendation of 22 December 2006 on Safe and Efficient In-vehicle Information and Communication Systems: Update of the European Statement of Principles on Human Machine Interface. Tech. Rep., Commission of the European Communities, 2007. Ackermann, J., Bünte, T., Odenthal, D., 1999. Advantages of active steering for vehicle dynamics control. In: 32nd International Symposium on Automotive Technology and Automation, pp. 263–270. Adell, E., Várhelyi, A., 2008. Driver comprehension and acceptance of the active accelerator pedal after long-term use. 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